The present invention relates generally to fault detection and correction in an alternating current (a.c.) to direct current (d.c.) conversion system. More particularly this invention relates to the detection of an improperly conducting controlled rectifier within a bridge circuit utilized to supply a d.c. load, for example a motor, with power from an a.c. source and to means for correcting or rendering nonconductive such improperly conducting rectifier. The present invention, although generally applicable to such systems, is particularly applicable to conversion systems employing digital type firing systems.
It is well known in the art to provide a system for the conversion of a.c. to d.c. using a full wave rectification bridge employing a plurality of controlled rectifiers which, at the present time, are usually thyristors of the type known as silicon controlled rectifiers (SCRs). It is also well known that whether the system be single phase or polyphase, the value of the d.c. voltage at the output of the rectifying bridge can be varied by controlling the phase firing angle of the individual bridge rectifiers. Phase angle control, as is well understood in the art, means that the individual rectifiers are fired at controllable times within the applied voltage cycle such that they are conductive only for a portion of the cycle. The use of such bridge circuits for the control of d.c. motors is also well known in the art and such systems normally employ a variety of feedback and control circuits to adjust the phase firing angle of the rectifier bridge so as to control motor operation through the control of the voltage applied to the motor. An example of such a system employing a current limit feature in an analog mode is shown and described in U.S. Pat. No. 3,526,819, "Current Limit For Motor Control Systems" by C. E. Graf issued Sept. 1, 1970. An example of a digital type of bridge control circuit may be found in U.S. patent application Ser. No. 927,795, filed July 25, 1978, "Power Converter Control" by W. B. Jarvinen which is a continuing application of Ser. No. 743,863, filed Nov. 22, 1976 (now abandoned). Both the patent and the application are assigned to the assignee of the present invention and are specifically incorporated hereinto by reference.
Controlled rectifier power conversion systems such as are here being discussed are susceptible to what are known as d.c. faults or "shoot-throughs". This is particularly true when the power conversion system supplies power to a motor and the motor is operating in the regenerative mode. A d.c. fault occurs when one or more of the rectifiers of the bridge are conducting at a time when they should be nonconducting. These faults or shoot-throughs may result from a variety of reasons including the failure of a particular rectifier to fire or conduct at its appointed time because of an intermittent faulty rectifier firing circuit connector or firing control. It may also be the result of the failure of one rectifier to commutate off its predecessor due to excessive current or lack of sufficient volt seconds due, for example, to a.c. supply line voltage reduction. Other possible causes include the false firing of one rectifier in the same a.c. supply phase as a rectifier properly fired due to excessive anode to cathode change of voltage with respect to time and electrical noise coupled into the firing circuit associated with the rectifiers. Regardless of the cause of the d.c. fault or shoot-through, the end result may be fuse failure and sudden shutdown of the drive or in a more exaggerated case the excessive currents may cause damage to the motor, if the motor is not otherwise protected.